1. Field of the Invention
The present invention relates to fuel cells and, more specifically, is a novel design that results in a scalable, lightweight fuel cell with a low operating temperature.
2. Description of the Related Art
The emerging changes in consumer and defense electronics have generated a complex demand on power sources. Use of conventional batteries, fuel cells, and other power generators, such as internal combustion engines, heavily restrict the range and application of modern electronic devices. In the area of transportation, the national need for energy is growing, and fossil fuel reserves are shrinking.
Batteries and fuel cells, once thought of as ideal solutions to national energy and environmental needs, have not been able to meet any projected goals. For example, the zero-emission standard set by California was based on unrealistic expectations of current technology for both batteries and fuel cells. It is now known that California cannot realize its zero-emission goal with current technology. Furthermore, current discussions are not just about zero-emission, but developing alternate sources of energy to substitute for the depleting fossil fuels.
Unless a new technology is developed within the next few decades, the modus operandi for obtaining fossil fuels could become far more complex than today. Investment of resources in alternate energy sources including development of fuel cells that operate closer to their theoretical efficiencies is a good long-term plan.
The reasons for the popularity of the fuel cell include its potential to use renewable energy sources, its ability to produce non-polluting energy, and its high conversion efficiency and high (theoretical) energy density. Forty years of intense work on fuel cells have been largely confined to reengineering old ideas, which go back to the last century.
Fuel cells remain a top priority because of the unparalleled promise they hold. To break the barrier between promise and reality, the power and energy densities need to be boosted by a factor of three or more, the wasted heat needs to be eliminated, and the amount of platinum (Pt) loading must be reduced by three orders of magnitude (from 4 mg to 4 μg), or better yet, Pt should be replaced with a cheaper and more abundant material.
Fuel cells are power sources analogous to batteries. The major difference between the two is in the storage of fuel. In a fuel cell, the fuel is stored outside the reaction chamber, similar to gasoline used by an automobile engine. (In a battery, the fuel is stored internally.) Therefore, unlike a battery, fuel cells generate power for extended periods of time, limited only by the availability of the fuel. The success of a fuel cell for practical applications is determined by two factors: 1) the design of fuel storage outside the fuel cell and the delivery of the fuel into the cell; and 2) the efficiency of the reactions of the fuel and oxygen on the catalytic surfaces of the electrodes.
The most promising fuel for fuel cells that operate near room temperature is hydrogen. The most efficient storage medium for hydrogen is methanol. Therefore, the methanol-air fuel cell is the most popular of all fuel cells. Like all other fuel cells, the methanol-air cell generates electricity through two separate electrochemical reactions as shown below and in FIG. 1:CH3OH+H2O→CO2+6H++6e−  (at the negative terminal)3/2 O2+6H++6e−→3 H2O  (at the positive terminal)In ordinary terms, methanol (CH3OH) is oxidized at the anode and oxygen (O2) in the air is reduced at the cathode to produce carbon dioxide (CO2), water (H2O) and electricity (e−) or CH3OH+3/2 O2→CO2+2H2O. The reactions at the negative and positive terminals are conducted on special types of catalysts made from two relatively expensive metals, namely platinum(Pt) and ruthenium(Ru).
FIG. 2 shows the conventional design of methanol-air fuel cells used in 5-100 W power generation. It is seriously limited by weight (33 g/W excluding the fuel), high operating temperature (100° C.), and high demand on the precious platinum (250 mg/W) and ruthenium (125 mg/W). However, if these limitations are overcome, a methanol-air fuel cell becomes one of the most attractive power systems because methanol has the single best storage capacity for hydrogen among all the hydrogen-based fuels.
The proton exchange membrane (PEM)-type methanol fuel cells use a proton conducting polymer membrane such as a NAFION®, a PFSA membrane, as the electrolyte. It is, as its name implies, a conductor of protons that are generated at the anode and consumed at the cathode. The PEM also allows the transport of water molecules that are generated at the cathode and partly consumed at the anode. In fact, for the NAFION® electrolyte to function as an effective electrolyte, it should always be kept hydrated. Unfortunately, the NAFION® membrane also transports methanol, whose molecules have similar physical properties as water.
Methanol crossover from the anode to the cathode has a depredatory effect on the oxygen cathode: methanol interferes with the oxygen reduction reaction, thus reducing the oxygen current and the fuel cell current. Methanol crossover can be partly alleviated by increasing the thickness of the NAFION® membrane, but this increases the internal resistance and, therefore, the internal resistive losses of the cell voltage. Increased internal resistance also increases the internal heat generated due to the current flow, which tends to dehydrate NAFION®, thus further contributing to the problem of heat generation.
NAFION®-117 is a compromise between the two opposing problems—resistance vs. methanol crossover—and is commonly used in fuel cells. In the absence of an alternative proton conducting polymer membrane to NAFION®, a methanol-tolerant oxygen cathode, a low current discharge, and a low operating temperature are the best alternatives to minimize internal polarization and methanol interference problems.
In the conventional design, one side of the NAFION® is coated with the cathode material and the other with the anode material. The cathode material is Pt powder on carbon support. The anode is a Pt-Ru powder (50% atomic ratio of platinum and ruthenium), also on carbon support. The Pt-loading on both electrodes is on the order of 1-10 mg/cm2. One carbon cloth placed on the top of each electrode provides a passage to the fuel (3% solution of methanol in water to the anode) and air (oxygen to the cathode). The carbon cloth also provides the electrical contacts to the electrodes.
On the top of the carbon cloth is a plate, also known as the “bi-polar” plate, that feeds both methanol and air into the fuel cell. Each plate is made from two sheets of machined graphite or titanium, held together by an electrically insulating material. One side is placed on a carbon cloth that is attached to anode-side (negative terminal) of a NAFION® membrane, and the other to the cathode-side (positive terminal) of yet another NAFION® membrane. Several such combinations of anode-NAFION®-cathode, sandwiched between carbon cloths and bi-polar plates are stacked to form a fuel cell.
As noted above, most conventional fuel cells are operated at about 100° C. with the minimum operating temperature of a fuel cell that also generates a useful amount of current being 60° C. At the 60° C. temperature, the cell generates about 200 mA/cm2, with a manageable degree of dehydration. Some fuel cells actually operate at much higher temperatures—some at 130° C.—if water can be carried in great abundance along with the fuel cell to re-hydrate the membrane. Yet, the fuel cell voltage is far from its theoretical value—0.4 V vs. 1.2 V—a loss of more than 0.8 V, mostly due to internal polarization. This difference manifests itself as internal heat, which is further compounded by the increasing resistance of the NAFION membrane at higher temperatures.
As a result of internal polarization and resistive heating, the fuel cell loses about two thirds of all the energy that it produces. To dissipate this heat, conventional designs use fans and radiators, which add to the weight of the fuel cell when, in principle, the weight of a fuel cell should not exceed the weight of the stack+fuel+fuel storage system. In fuel cells that generate 10W or more power, the weight of the accessories easily exceeds the weight of the stack.
Furthermore, the stack cannot tolerate more than a 3% wt. of methanol solution in water as the fuel. That means, 97 grams of water should be added to every 3 grams of methanol, although only 1.69 grams of water participates in the reaction. Most fuel cell systems store and feed the fuel in the 3% wt. form, under the assumption that the excess water is utilized to hydrate the (NAFION®) electrolyte. The result of the additional water and accessory requirements is a heavy fuel cell that is only able to produce a power of 33 W/kg. This power density is paltry when compared with its theoretical power density of 2,413 W/kg, and the power densities of batteries at 75-100 W/kg.
Additionally, if the present practice of platinum “loading” of the catalyst (4 mg/cm2) continues, then the cost of the fuel cell will remain prohibitively high. It is unlikely to be a ubiquitous source of power, or used with disposable, field-deployable sensors and unmanned vehicles. Besides, at the 4-mg/cm2 level of loading, there will not be enough platinum left for its use as a catalyst in any large-scale operation.
The list of limitations of methanol-air fuel cells, based on conventional designs, can be summarized as follows:                Methanol interference on the cathode minimizes oxygen current, therefore, the cell current.        High temperature (≧60° C.) is necessary to generate 200 mA/cm2, but this contributes to methanol crossover and NAFION® dehydration.        High temperature operation demands the availability of excess water and requires fans and radiators to dissipate heat, which add to the weight of the fuel cell.        Fuel-feed bi-polar plates are machined from graphite or titanium, which is one major source of weight. A lightweight alternative is not yet used in fuel cells of 20 W or higher power.        High internal resistance and polarization losses generate >100 W heat in a 20 W fuel cell, most of which is wasted through radiation.        The concentration of methanol inside the fuel cell can only be about 1 M (3% solution in water). Most fuel cells also use the same methanol/water ratio in the storage. That means, to generate 20 W continuous power for 3 days will require 2.5 kg of the fuel.        High platinum “loading” results in a prohibitively high cost for the fuel cell.        
All of these limitations make a methanol-air fuel cell based on conventional designs unattractive.